These materials being of high volatility and being liquid only at low temperatures and/or high pressures present special problems in handling.
In particular, although need for pressure equalization has been realized as between various areas of the systems, recycling of vapour arising from the liquid cryogenic material has previously been done through ducting quite separate from that which conducts liquid between the various areas of the systems, using separate return pumps. This is wasteful in terms of constructional material needed, in terms of compactness and in terms of inefficiency in the necessary prevention of heat absorption into the system.